Conventionally, a linear motor is used for a system required to have ultraprecise positioning/high thrust, such as a conveyance system typified by that of tooling equipment or that of semiconductor-fabrication equipment. An armature that is a constituent element of the linear motor is classified roughly into a coreless type having no core and a core type having a core.
The coreless type linear motor of these is structured as shown in FIG. 1. FIG. 1 is a perspective view of the whole of the coreless type linear motor used conventionally, which is identical in the general structure to the present invention described later. FIG. 5 is a view showing a cooling structure of a coreless type linear motor according to a first conventional technique, which corresponds to a front sectional view along line A—A of FIG. 1. The linear motor used in this example is of a moving-coil type in which an armature serves as a rotor, and a permanent magnet for a field system disposed between magnetic air gaps on both sides of the armature serves as a stator, and the linear motor has a magnetic-flux penetration structure.
In the figures, 1 is the linear motor, 2 is a field yoke, 3 is the permanent magnet, 4 is a yoke base, 5 is the armature, 6 is an armature coil, 9 is a resin mold, 13 is a winding-fixing frame, 14 is a frame, and 15 is a refrigerant passage.
In the linear motor 1, a plurality of permanent magnets 3 that form field poles are linearly arranged on the side faces of the two-line field yokes 2 so that the polarities of the N and S poles alternately form, and the yoke base 4 is disposed between the field yokes 2, thus forming a stator. Further, in the linear motor 1, the armature 5 is disposed in parallel with the linearly arrayed permanent magnets 3 with a magnetic air gap between the armature and the permanent magnets. The armature 5 has two lines of coreless type armature coils 6 formed by tabularly molding a plurality of coil groups, and the armature coils 6 are linearly arranged on both sides of a winding-fixing frame 13 made of metal, such as stainless steel, and the winding-fixing frame 13 and the armature coils 6 are bonded together with a resin mold 9.
Further, a cross-sectionally T-shaped frame 14 used to fix the armature 5 and made of metal, such as stainless steel, is provided on the upper part of the armature 5 along its longitudinal direction. After an end of the winding-fixing frame 13 is inserted into a recessed part 14a of the frame 14, the armature 5 and the frame 14 are bonded together by brazing, welding, or gluing. A refrigerant passage 15 to which a refrigerant is supplied from a refrigerant supplying device, not shown, is provided in the interior of the frame 14. A linear guide made up of a slider and a guide-rail, not shown, is attached to each of the rotor and stator sides of the linear motor, whereby the rotor can be moved linearly with respect to the stator.
In this structure, when an electric current is applied to the armature coil 6 of each phase from a power source not shown, an electromagnetic force works in the longitudinal direction of the armature coil 6 from a magnetic field produced in the magnetic air gap between the armature coil 6 and the permanent magnets 3 by an electromagnetic action thereof with the permanent magnets 3, and a thrust is generated, thereby performing a smooth linear movement. When a driving current to drive the rotor flows through the armature coil 6 at this juncture, the armature coil 6 generates heat because of internal resistance, but the heat generated by the armature coil 6 is transferred to the frame 14 through the resin mold 9 and the winding-fixing frame 13, and a heat exchange is performed by a refrigerant circulating through the interior of the refrigerant passage 15, and the whole of the armature 5 is cooled.
In contrast with the coreless type linear motor mentioned above, a core type linear motor is structured as in FIG. 11. FIG. 11 is a front sectional view of the core type linear motor according to a second conventional technique. In this example, a description will be given of a moving coil type linear motor that uses an armature as a rotor, in relation to the second conventional technique and the present invention described later.
In FIG. 11, 21 is a fixed base, and 22 is a linear guide that is made up of guide rails 22A provided at right and left ends on the fixed base 21 and sliders 22B corresponding to the guide rails 22A, respectively. 23 is a linear motor, 24 is a stator, 25 is a rotor, 26 are tabular field yokes, each facing the fixed base 1 and fixed thereto, and 27 is a plurality of permanent magnets arranged along the field yoke 26 (i.e., vertically with respect to the page surface) so that alternately different polarities form. The stator 24 is formed by field poles produced by the field yokes 26 and the permanent magnets 27. 28 is a core that faces the permanent magnets 7 with a magnetic air gap between the core and the magnets and that is formed by laminating electromagnetic steel sheets in the height direction of the permanent magnet 7, and 29 is a coil wound in the interior of a slot 8A of the core 28 and fixed with a resin mold not shown. The rotor 25 is formed by an armature made up of the core 28 and the coil 29. 30 is a table used to mount a load. When the core 28 is fixed to the table 30, a fixing bolt 31 is passed through a through hole 28B made in the core 28, and then the fixing bolt 31 is screwed into a female screw part 30C made in the table 30, whereby the core 28 and the table 30 are fastened together.
Since the linear motor 23 has a structure in which each permanent magnet 27 is disposed as if to squeeze both sides of the coil 29, and the lines of magnetic force of the permanent magnets 27 penetrate through the coil 29, i.e., the linear motor 23 has a so-called magnetic-flux penetration structure, a sidewise force is not applied to the table 30 without generating an attraction force between the permanent magnets 27 and the coil 29.
However, the coreless type linear motor according to the first conventional technique has the following unresolved problems.    (1) The thermal conductivity of the resin mold 9 with which the surroundings of the coil group are covered and the thermal conductivity of the winding-fixing frame 13 that holds the armature coil 6 are much smaller than that of the armature coil 6. Additionally, since the contact area of the winding-fixing frame 13 with the frame 14 is small, and the winding-fixing frame 13 is not in direct contact with the refrigerant passage 15, the thermal resistance of a path from the armature coil 6 to the refrigerant passage 15 is large on the whole. That is, since the cooling capacity of the linear motor depends on the thermal resistance from the armature coil 6 to the refrigerant passage 15, measures to cause a refrigerant to merely flow toward the frame 14 have a limit, and heat caused by a rise in temperature of the armature coil 6 could not be efficiently radiated.    (2) If heat caused by a rise in temperature of the armature coil 6 cannot be efficiently radiated to the frame 14 as mentioned above, the internal resistance of the armature coil 6 increases, and a driving current decreases in correspondence with the temperature rise of the armature coil 6. In this situation, a decrease in the driving current has brought about a significant decrease in the thrust of the rotor of the linear motor 1 because the thrust of the rotor of the linear motor 1 is proportional to the driving current.    (3) Generally, if the temperature of the armature coil 6 rises high as mentioned above, the resin mold 9 covering the armature coil 6 undergoes thermal deformation in correspondence with the temperature rise caused by the heat generation of the armature coil 6, whereas the frame 14 through the interior of which a refrigerant flows is small in a rise in temperature and does not undergo thermal deformation. Therefore, there has been a concern that a distortion will occur between the resin mold 9 and the frame 14, and the resin mold 9 weak in strength will be damaged. Additionally, in a situation where the linear motor is used as a stepper driving mechanism of, for example, semiconductor-fabrication equipment operated in a vacuum environment, if the temperature of the resin mold 9 rises because of the heat generation of the armature coil 6, gas is emitted from the surface of the resin mold 9, and the vacuum environment needed for a manufacturing process is contaminated, and, as a result, little reliability has been placed on the cooling system of the linear motor.    (4) Additionally, under the method of inserting an end of the winding-fixing frame 13 into the recessed part 14a of the frame 14 and fixing them together, a length by which the winding-fixing frame 13 is inserted into the frame 14 is short as shown in FIG. 5, and therefore an unsolved problem exists in the fact that the unified structure of the armature 5 and the frame 14 lowers the rigidity of the rotor. As a result, when the rotor was run, the rotor generates vibrations toward the stator facing the rotor with a magnetic air gap therebetween, thus lowering the movement precision.    (5) Additionally, as another conventional technique, a proposal has been made of a linear motor, not shown, structured to dispose a plurality of heat sinks used to flow a refrigerant between adjoining coils formed in the same armature coil row, but, since the heat sinks are externally situated outside the winding-fixing frame fixing the armature coil, the whole of the armature becomes large in size, and the number of assembly steps increases so as to make a size reduction not possible if they are covered with a resin mold.
The core type linear motor according to the second conventional technique also has the following unsolved problems.    (1) In the moving coil type linear motor 23 shown in the figure, if a driving current continues to be supplied from a power source, not shown, to the coil 29 in order to raise the thrust of the motor, the temperature rises in correspondence with an increase in the internal resistance of the coil 29, and the heat value increases. Therefore, disadvantageously, the heat emitted from the coil 29 is transferred to the table 30 fixed to the upper part of the core 28 through the core 28, and the core 28 and the table 30 undergo thermal deformation. Especially, in a part facing the fixed base 21 of the core 28, a warp in the longitudinal direction caused by the thermal deformation becomes larger proportionately with a rise in temperature of the coil 29 with the lapse of time.
Not only the moving coil type linear motor but also a moving magnet type linear motor, not shown, that uses field poles as rotors has the following common problems.    (2) If the coil 29 of the linear motor 23 greatly generates heat, the core 28 is correspondingly greatly deformed in the direction facing the arrayed permanent magnets 2, and a magnetic air gap between the coil 29 and the permanent magnets 27 often varies, thus generating a cogging thrust. As a result, the driving performance of the linear motor 23 has deteriorated proportionately with an increase in the cogging thrust, and a great influence has been exerted especially on the processing precision.    (3) On the other hand, thermal deformation caused by transferring the heat to the table 30 exerts a bad influence on the slider 22B attached to the table 30 or on a scale used for a linear encoder not shown, so that an error in positioning accuracy occurs, and it has been difficult to achieve highly accurate positioning.    (4) In the linear motor 23, a resin mold (not shown) covering the coil 29 undergoes thermal deformation through the influence of a rise in temperature of the coil 29, and thereby the resin mold has been damaged.    (5) If the resin mold is broken, for example, when the linear motor 23 is used in a vacuum environment, the amount of dust caused by gas emitted from the surface of the resin mold will increase, and the vacuum environment will be impaired, whereby little reliability has been placed on the linear motor 23.
The present invention has been made to solve the aforementioned problems, and it is a first object thereof to provide a linear motor, which is of a coreless type, capable of preventing the thrust of a rotor from decreasing in accordance with a rise in temperature of an armature coil by efficiently transferring the heat of the armature coil to a frame, the linear motor having a small size, high rigidity, and high reliability.
It is a second object of the present invention to provide a linear motor, which is of a core type, having high accuracy and high reliability by keeping a core and a table from undergoing thermal deformation.